Characterizing the transfer of bacterial antibiotic resistance genes across generations of swine

by 1977- Garner, Kimberly Nakia

Abstract (Summary)

Sows and pigs were used to characterize the origin, transfer and persistence of bacterial
resistance in swine. Effects of sow’s previous exposure to antibiotics and subsequent use
of antibiotics in their pigs on antibiotic resistance of Salmonella enterica Typhimurium,
Enterococcus faecalis, and E. coli were determined. Eight pregnant sows were divided
into two groups, with four sows receiving oxytetracycline and four sows receiving no
antibiotics. Fecal samples were obtained from sows prior to antibiotic exposure, and at 1-
week intervals until pigs were weaned. Weaned pigs were challenged with Salmonella
Typhimurium containing a nalidixic acid. Pigs from each sow treatment group were
divided equally between a subtherapeutic antibiotic treatment regimen or exclusion of
antibiotics. Pigs on the antibiotic treatment received apramycin at 150 g/ton of feed,
beginning 7 days postweaning and lasting for 14 days, followed by oxytetracycline at 50
g/ton throughout the grow/finish period. Fecal samples were obtained from the pigs
while on the sows and at 2, 7, 14, 30, 60, 114 and 115 days postweaning. The Salmonella
challenge organism, E. coli and E. faecalis were recovered and tested against both
apramycin and oxytetracycline using a minimum inhibitory concentration (MIC) analysis.
Data were analyzed using the mixed models procedure of SAS. Polymerase Chain
Reaction and transformation techniques were used to characterize genetic resistance
elements and determine if the location of such gene sequences. Random apramycinresistant
E. coli isolates (n = 110) were chosen from antibiotic treated sows and pigs,
non-antibiotic treated sows and pigs and environmental manure to test through PCR,
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plasmid profiling, and macrorestriction analysis. Treatments affected antibiotic
resistance to the greatest extent in E. coli, compared to Salmonella Typhimurium and
Enterococcus faecalis. The greatest resistance to apramycin occurred in E. coli isolates
from nursing pigs on sows that had earlier exposure to tetracyclines, and from pigs
treated with apramycin during the postweaning period. Resistance to oxytetracycline was
consistently high throughout the study in isolates from all pigs and sows, including those
with no previous exposure to that drug. Genes responsible for apramycin resistance were
found in approximately 90% of resistant isolates and their location was determined to be
on bacterial plasmids. It was also determined that several different types of E. coli
contained the aac(3)-IV gene responsible for apramycin resistance. These results
indicate that apramycin and tetracycline resistance in E. coli was affected by previous use
of tetracycline in sows (P ? 05). Additionally, subsequent use of antibiotics in pigs also
affected (P ? 05) resistance levels in E. coli, whereas Salmonella Typhimurium and
Enterococcus faecalis were not affected by antibiotic use in sows or pigs.
Key Words: antibiotic resistance, swine, E. coli
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